1. Field of the Invention
This invention relates generally to satellite communication systems and, more particularly, to an error correction encoding system for use in an in-flight programmable spacecraft.
2. Discussion of the Related Art
Modern spacecraft routinely exhibit useful lifetimes in excess of ten years. This extended operating life poses a problem for the designers of satellites which transmit digital data that either originated on board or is being retransmitted after on-board demodulation. Rapid advances in coding theory and applicable technology has revolutionized satellite transmission and comparable advances are anticipated in the future. However, once the spacecraft is launched, the downlink encoding scheme cannot be changed.
Today, an on-board computer may be used to perform encoding aboard some spacecraft. Although a software-based encoder might be used for certain types of encoding, it would require a processor architecture that was optimized with respect to both the logic operations associated with encoding and the spacecraft's operational environment (especially temperature and radiation). In other words, the computer-based approach is not suited to high speed data transmission because of processing speed limitations and power penalties imposed by the execution of software instructions and the processor's input/output structure.
Therefore, it is desirable to provide a technique which allows the satellite designers and operators to exploit improved coding techniques for high speed data transmission and reductions in ground terminal decoding costs that may occur after launch. In particular, the present invention permits the satellite designers and operators to change the downlink encoding scheme after the spacecraft had been launched. This enables the future use of coding schemes whose decoding at the receiving terminal was considered impractical at the time of launch, coding schemes that were not developed until after the satellite had been launched or, if it is determined that operating conditions permit, coding schemes simpler than originally deployed.
Over the lifetime of the satellite, the processor costs (i.e., MIPS/dollar) will continue to diminish. As these costs fall, computationally intensive decoding algorithms that initially could not be implemented in an acceptable form will become more attractive. The system designers will then be able to exploit future coding performance improvements in several ways, including: (1) improve overall system availability (i.e., the percentage of time at which the downlink performs at or below a specified bit error rate), (2) increase average throughput by reducing the ratio of coding bits to data bits, and (3) maintain current performance levels while operating terminals with smaller reflectors.